Hello.
the behaviour about vm image based provisionning had change between 1.13 and 1.14 version with vmware provider.
I dont know if it failed with an other provider.

Previouly, with 1.13 :
i had a template with only one disk and reach to provisionned a VM with 2 disk.

Now with 1.14 :
the number of disk in template must be the same than the number of disk for the future provisionned vm.

It failed with this error:
Failed to create a compute Vmware-PProd (VMware) instance machinename.example.com: InvalidRequest: Error parsing string 2 as enum type VirtualDeviceConfigSpecOperation while parsing serialized value of type vim.vm.device.VirtualDeviceSpec.Operation at line 1, column 4223 while parsing property "operation" of static type VirtualDeviceConfigSpecOperation while parsing serialized DataObject of type vim.vm.device.VirtualDeviceSpec at line 1, column 4174 while parsing property "deviceChange" of static type ArrayOfVirtualDeviceConfigSpec while parsing serialized DataObject of type vim.vm.ConfigSpec at line 1, column 577 while parsing property "config" of static type VirtualMachineConfigSpec while parsing serialized DataObject of type vim.vm.CloneSpec at line 1, column 325 while parsing call information for method CloneVM_Task at line 1, column 177 while parsing SOAP body at line 1, column 167 while parsing SOAP envelope at line 1, column 0 while parsing HTTP request for method clone on object of type vim.VirtualMachine at line 1, column 0

Reproducte way :
From foreman try to create a new vm (by cloning or from a template) with a number of disk different than the source.

Thank you for your attention.